J Sol-Gel Sci Techn (2006) 40:359–363 DOI 10.1007/s10971-006-8986-2

Distributed feedback multipeak laser emission in Rhodamine 6G doped organic-inorganic hybrids D. C. Oliveira · Y. Messaddeq · K. Dahmouche · S. J. L. Ribeiro · R. R. Gonc¸alves · A. Vesperini · D. Gindre · J. -M. Nunzi

Published online: 22 August 2006 C Springer Science + Business Media, LLC 2006


Abstract Organic-inorganic hybrid materials were prepared from an ureasil precursor (ureapropyltriethoxysilane designated as UPTES) and acrylic acid modified zirconium (IV) n-propoxide. Thin films containing rhodamine 6G (Rh6G) were prepared by spin-coating on glass substrates with different Zr:Si molar ratios (Zr:Si = 75:25, 50:50 and 25:75). Refractive index, thickness, number of propagating modes and attenuation coefficient were measured at 543.5, 632.8 and 1550 nm wavelengths by the prism coupling technique. Distributed feedback (DFB) laser effect was observed and studied as a function of films thickness and refractive index. Keywords Organic-inorganic hybrids . DFB lasers . Waveguides 1. Introduction Organic-inorganic hybrid materials potentially suitable for production of optical waveguides and functional integrated optic devices have been prepared in the last years. One of the utmost interesting characteristics is the easy incorporation of D. C. Oliveira · Y. Messaddeq · K. Dahmouche · S. J. L. Ribeiro (
) Institute of Chemistry- UNESP, CP 355, 14801-970 Araraquara, SP, Brazil e-mail: [email protected] R. R. Gonc¸alves Department of Chemistry- FFCLRP- USP- Ribeir˜ao Preto, SP, Brazil A. Vesperini · D. Gindre · J.-M. Nunzi Laboratoire des Propri´et´es Optiques des Mat´eriaux et Applications UMR-CNRS 6136, Universit´e d’Angers, 49045, Angers, France

organic dyes [1–7]. Sensors, dye lasers, photochromic, nonlinear optics and photovoltaic devices are some of the possible applications for these dye containing organic-inorganic materials [see Review in 3, 4]. Concerning laser emission Avnir et al. [8] first proposed sol gel derived glasses for the incorporation of laser dyes. After this first proposal several other papers and reviews have appeared in the literature [3, 4]. Recently Nhung et al. [9] have studied the energy transfer between Rhodamine B (RhB) and Perylene Red dyes. A laser emission in solid samples was obtained from a methyl modified alcoxide [MTEOS- (Me–Si(OEt)3 ] by using a stable plano-concave linear cavity. Besides this activity concerning lasing in organicinorganic hybrids, some of us have studied laser emission in Distributed Feedback lasers composed by polymeric films containing organic dyes [10, 11]. The principle of distributed feedback lasers was first described by Kogelnik and Shank [12]. In these mirrorless lasers, the optical feedback is obtained by Bragg scattering from periodic perturbations of gain and/or refractive index of the medium. The optical feedback and gain are distributed throughout the medium and the emitted laser wavelength depends on the modulation period. In [10–14] dye doped polymeric films have been studied. Zhu and Lo [15] have shown DFB emission in rhodamine 6G doped titania-silica inorganic films. An optically pumped dynamic distributed feedback scheme was utilized and the waveguiding structure was shown to reduce the laser emission threshold. Moreover the number of laser modes was observed to increase with the film thickness. In this paper we present results for DFB emission in organic-inorganic hybrids that some of us have studied before [6, 7]. Hybrid hosts formed by polyether-based chains of variable length grafted at both ends to a siliceous backbone through urea cross linkages (termed as di-ureasils) have been prepared containing metacrylate modified zirconium Springer

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propoxide. The hybrid structure was elucidated by scattering and spectroscopic techniques. Zirconium based nanoparticles were identified in the di-ureasil host with increasing quantities following the increase in the zirconium content. Samples were observed to be UV photosensitive and preliminary experiments have shown the possibility to obtain diffraction gratings and channel waveguides through light patterning [7]. These new materials display therefore interesting tunable properties that could be interesting considering the DFB emission. In this work we have studied the waveguide properties of films obtained with different Zr4+ contents. In addition the incorporation of Rh6G and deposition of thin films on glass substrates by spin-coating have led to the luminescent films from which DFB laser emission was observed.

2. Experimental The base host matrix (di-ureasil) contains short oxyethylene polymer chains grafted to a siliceous network by means of urea linkages. The cross-links between the organic and the inorganic components were formed by reacting the NH2 groups of a diamine (α,ω-diaminepoly (oxyethylene-co-oxypropylene known commercially as r ) with the –N=C=O group of 3Jeffamine-ED600
isocyanatepropyltriethoxysilane in tetrahydrofuran (THF), under reflux at 80◦ C for 6 hours. The UPTES precursor was isolated after the evaporation of the solvent. The prepared hybrid samples were called d-U(600)-mZPO where d-U(600) refers to the di-ureasil (600 represents the average molecular weight of the polymer chains corresponding to approximately 8.5 oxyethylene repeat units) and mZPO refers to the metracrylic acid modified zirconium alcoxide obtained by mixing in buthanol Zirconium n-propoxide and methacrylic acid (H2 C=C(CH3 )CO2 H) in a molar ratio 1:1. Stable hybrid sols were prepared with varying Si:Zr molar ratios (Si:Zr = 75:25, 50:50, 25:75). Pre-hidrolysis was performed separately for both precursors with HCl 0,1 M aqueous solution and the final sols were obtained by mixing different amounts of each precursor. Films (doped and non doped compositions) were obtained by spin-coating technique (1000 rpm, 30 s) on clean sodalime glass substrates. Samples were treated at 90◦ C for 30 min between each deposition. The number of layers was chosen in order to obtain monomode and multimode waveguides. For the doped compositions Rh6G was added at a concentration of 4 × 10−3 mol·L−1 . The refractive index and the thicknesses of the waveguides were measured for both transverse electric (TE) and transverse magnetic (TM) polarization by an m-lines technique (Metricon mod. 2010). The apparatus was equipped with Si and Ge detectors to collect the visible and NIR respectively. Springer

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Two He-Ne lasers, operating at 632.8 and 543.5 nm and one diode laser operating at 1550 nm were employed. The resolution in the determination of the angles corresponding to the propagation modes was 0.0075◦ . In order to assess the propagation losses of planar waveguides, light power scattered out of the waveguide plane, which is proportional to the guided power, was recorded by a scanning optical fibre probe moving along the direction of propagation of the light. The attenuation coefficient was obtained by fitting the data with an exponentially decaying function, thus assuming a homogeneous longitudinal distribution of the scattering centres and constant loss parameter of the planar waveguide; in-plane diffraction effects were ignored, together with those of any existing mode coupling, in the case of multimode waveguides. The measurements were performed by excitation of the TE0 mode of the planar waveguide at 632.8 nm and 1550 nm wavelenghts. DFB laser emission was measured with a dynamic grating obtained with a Lloyd mirror interferometer set-up schematically shown in Fig. 1. A frequency doubled Q-switched mode-locked Nd:YAG pulsed laser (532 nm, 1 Hz, 35 ps pulse duration) was used as pump source. The beam was focused n the sample with a cylindrical lens. One part of the incident beam reaches directly the film while the other part overlaps the first one after reflection on the mirror. An interference pattern is created on the sample, and gain and refraction index can be modulated. The grating period  corresponding to the space between fringes in the interference pattern is given by =

λp 2 sin α

(1)

where λp is the pump wavelength and α is the angle between the pump incident axis and the mirror plan. The Bragg condition [14] selects the propagation constant β of the waveguided modes undergoing DFB laser emission. β is related to the grating period  by the angle α: β=

2mπ sin α n eff λ p

(2)

where neff is the material effective refractive index and m is the Bragg reflection order. The Bragg condition determines the laser emission wavelength and the β-induced wavelength must fall inside the stimulated emission bandwidth to be amplified. The light emited by the sample was collected by an optical fiber and transmited to a Huet M25 spectrometer equiped with a Hamamatsu C4547-95 CCD camera. 3. Results and discussion Stable and homogenous sols were obtained for all compositions. Structural features for xerogels have been discussed

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Fig. 1 Lloyd mirror interferometer setup. The cylindrical lens focuses the beam to obtain an interference area along a narrow stripe on the hybrid film

in our previous work from X-ray diffraction (XRD), small angle X-ray scattering (SAXS) and 29 Si and 13 C nuclear magnetic resonance results [6, 7]. An effective interaction between zirconium-based particles and the siliceous nanodomains was proposed. Waveguiding properties were observed and Table 1 summarizes the results obtained from mlines measurements. Refractive index values increase from 1.508 (Zr:Si = 25:75) to 1.557 (Zr:Si = 75:25). For refractive index values lower than the one of the sodalime glass substrate highly reflective silicon substrates have been used. The number of propagating modes depends on the thickness, refractive index and wavelength of the light. Monomode waveguides can be easily obtained by controlling these parameters. Optical loss includes absorption and volume/surface roughness scattering contributions. Optical losses was estimated at 2–3 dB cm−1 . Figure 2(I) shows rhodamine emission spectra obtained from samples doped by Rh6G with compositions (Zi:Si = 25:75 and 75:25) under excitation of the 5145 Å Argon laser line. The typically broad band spectrum extending from 550 up to 720 nm with 40 nm full width at half maximum (FWHM) bandwidth can be observed. With the increase of the zirco-

nium content a broadening of 20 nm of the spectrum is observed reflecting the interaction between the dye molecule and the host. Figure 2(II) shows three spectra obtained with the DFB set-up. For energy values below 0.9 µJ/pulse only the Amplified Spontaneous Emission (ASE) is observed with a 10 nm spectrum bandwidth. At energy pulse of 0.9 µJ a narrow laser emission is observed superimposed on the ASE background. For higher energy pulses only the laser line is observed. The grating period, and therefore the laser emission wavelength is easily tuned with the Lloyd setup according to Eq. (1). Figure 3 shows typical spectra obtained for the sample with Zr:Si = 50:50. The figure inset shows that the observed laser emission wavelengths are in accordance with the calculated ones considering the cavity (period of the grating) longitudinal mode. In general the wavelength

Table 1 Optical parameters measured @ 543.5, 632.8 and 1550 nm for TE polarization for d-U(600)-mZPO planar waveguides on sodalime glass substrates substrate (n = 1.5209 @543.5 nm; 1.5176 @632.8 nm;1.5040 @1550 nm) Waveguide (Zr:Si) Thickness ( ± 0.1 µm) Refractive index ( ± 0.001)[number of guided modes] @543.5 nm @632.8 nm @1550 nm Attenuation coefficient ( ± 0.5) (@632.8 nm)

75:25 2.5

50:50 3.4

25:75 1.9

1.557-[4] 1.552-[3] 1.534-[1] 3.0

1.535-[3] 1.529-[2] 1.519-[1] 2.6

1.514-[0] 1.508-[0] –

Fig. 2 (I)- Emission spectra (λEXC - 5145 Å) for the Zr:Si = 25:75 (solid line) and Zr:Si = 75:25 (dashed line) samples (rhodamine 6G − 4.10−3 mol·L−1 ). (II)- DFB emission spectra- (a) pump power 0.9 µJ)

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Fig. 3 Laser output spectra as a function of the pump laser incidence angle for DFB laser action. Sample Zr:Si = 25:75. The inset shows that the observed laser wavelengths (symbols) agree with the values calculated for the cavity (grating period) longitudinal mode (dashed line)

modes (Table 1). The number of transverse modes which are guided by the film can be observed in the laser emission. Figures 4 and 5 show the increase in the number of observed modes with the increase in the refractive index and thickness respectively. For samples with refractive index lower that the one of the substrate only the longitudinal mode is observed. Therefore, with the increase in the thickness also the waveguided modes contribute to the laser emission. In fact

Fig. 4 Laser emission for samples with the same thickness (1.5 µm). (a) Zr:Si = 50:50 (n = 1.529 @ 632.8 nm)); (b) Zr:Si = 75:25 (n = 1.552 @ 632.8 nm)

tunability covering the entire dye estimulated emission if observed for all samples. With the increase in either the refractive index or the thickness an increase is observed on the number of guided Springer

Fig. 5 Laser emission for samples with the same refractive index and increasing thickness (a) 1.0 µm; (b) 1.5 µm; (c) 3.0 µm

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as theoretically studied before [12] for polymer films, the spectral structure of the DFB laser emission in these hybrids is a signature of the transverse modes waveguided into the film. The number of the observed laser modes agrees well with the number of waveguiding modes obtained through the m-lines spectroscopy.

4. Conclusion Organic-inorganic hybrids were prepared from ureapropyltriethoxysilane and acrylate modified zirconium(IV) propoxide precursors. Zr:Si molar ratio was in the range 75:25 to 25:75. Sodalime glass substrates were spin coated and the planar waveguides so obtained had the optical parameters (refractive index, thickness, number of propagating modes and attenuation losses) determined by m-lines spectroscopy. Rhodamine doped sols were also prepared and thin films were analysed as DFB lasers with a Lloyd mirror set-up. Tunability was easily observed with the laser emission covering the dye emission profile. Together with the longitudinal laser mode, transverse modes due to the films waveguiding properties were also observed. The number of waveguided modes was observed to increase with the thickness and refractive index of the films. The same number of guided modes is observed either in the DFB emission experiments or in the m-lines spectroscopy. Besides the intrinsic interest in multilines laser emission for integrated optics the DFB experiment could be used for the study of optical constants of luminescent hybrid films.

363 Acknowledgments Financial support of the Brazilian agencies FAPESP, CNPq and CAPES and also of the French agency CNRS is acknowledgded.

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